MASCOT May-June 2003

How does it work? No 3 - The Rear Axle and Differential

The Tool Chest

"What do you do if your rear axle is noisy?" he asked. "Turn up the radio" was the glib retort. "But what if the car doesn't have a radio?" No answer was the loud reply!

So what, exactly, is the rear axle for and what does it do? Well, it has three main functions - first, to receive the power from the engine (via the gearbox and propeller shaft) and turn it through 90 degrees to the wheels; second, to provide an overall reduction gear, usually about 4 : 1, which saves having to make this reduction in the gearbox, thus keeping the gearbox much smaller and enabling the straight-through top gear ratio to be employed; and third, to enable the wheels to travel at different speeds whilst cornering, without losing the drive.

The first two functions are carried out by the crown wheel and pinion, or in some cases, a worm and wheel, although these are relatively expensive and tend to be used more on heavy commercial vehicles: Worm, gears can give a large reduction in a small space. but the action at the teeth is all sliding. so they are prone to wear and run very hot. Fig 1 shows a typical Worm Drive arrangement.

There are three main types of crown wheel and pinion. These are - Straight Bevel, Spiral Bevel and Hypoid Bevel - see Figs 2, 3 & 4 resp. Although the Straight Bevel was cheap to produce and mechanically efficient, it was inherently noisy. This was reduced by the use of a helical form of tooth known as the Spiral Bevel. Quieter yes, but because they are angled to each other, the tooth pressures are much higher and the ordinary high viscosity gear oil used for the straight bevel gears is not good enough. The oil film breaks down under the high loads, causing rapid wear and scoring. Special oils were developed to lubricate the surfaces after rupture of the oil film and these are known as EP (Extreme Pressure) lubricants. They contain various additives such as sulphur, chlorine and phosphorous, which chemically react with the metal surfaces at high temperatures, to form a compound of low frictional resistance.

The Spiral Bevel also means that the thrust on the pinion is towards the centre on over-run. This is why, if your rear axle is moaning at you under load, the noise will sometimes go away on over-run (or vice-versa). A more robust bearing arrangement is therefore required than for the Straight gears, where the thrust is always outwards, as by drawing the pinion further into mesh, wear in the bearings can result in seizure. It is therefore important that there is no end float in the pinion bearings, and taper roller bearings are now used in the place of the early roller thrust bearings. These are preloaded to ensure that there is no end float even under load. The Hypoid Bevel is the more popular form in use today. The pinion axis is offset from the centre line of the crown wheel, generally below, which gives a lower propeller shaft and reduction in tunnel height. An offset of 1/5th crown wheel diameter is often used. You can see from Fig 4 that by lowering its axis, the pinion tooth pitch is increased and so a larger pinion diameter can be used, which gives a much stronger gear.

On crown wheels and pinions the tapered teeth are machined on case-hardened steel gears. These are then ground together to form a 'matched, or mated, pair'. It is therefore very important that engagement of the teeth is correctly set. Adjustment is usually by shims or screwed rings and is checked using 'Engineer's Blue' on the pinion teeth, which, when turned in the direction of rotation, should give the correct marking on the crown wheel as shown in Fig 5. Of course, moving the pinion in or out will affect the backlash, which is usually around 0.006". This is adjusted by moving the crown wheel towards or away from the pinion, and should be checked after each pinion setting using a clock gauge.

The third function is carried out by the Differential Gears. As you can see from Fig-6a, when a vehicle is cornering, the outer wheel must travel further than the inner wheel. If the wheels are interconnected the tyres will have to 'scrub' and the vehicle will tend to go in a straight line. The solution to this problem came in 1827, when Pequeur of France invented the differential. This mechanism allows the wheels to rotate at different speeds, but still maintains a drive to both wheels. The principle of this is shown in Fig 6b.

Consider the two discs to be linked by shafts to the wheels and interconnected with a lever. When a force is applied to C at the centre of the lever, each disc will receive an equal share. Movement of the discs will depend on the resistance opposing the motion of the shafts, so if a larger resistance acts on disc 8, the lever will tilt and push disc A forward a greater amount. A plan view of this is at Fig 6c.

In Fig 6d the discs and levers are replaced by bevel gears, which are called 'sun wheels' and 'planets' respectively. The drive is applied to the cross-pin and will push the planet gears forward to exert an equal torque on each sun wheel irrespective of their speeds. When the vehicle turns a corner, the inner wheel slows down and causes the planets to rotate on their own axis, speeding up the outer wheel.

A complete differential is shown at Fig 6e. The crown wheel is bolted to the differential cage, which supports the sun wheels and transmits the drive to the cross-pin.

And that's all there is to it, folks! TC